The present invention relates to a multifunctional coating technology that allows one to utilize a laser for printing such that the print can be designed to be either authenticatably eye visible or only fluorescently visible or totally invisible. This printing method uses a medium to high power density laser beam as the means for printing on a properly treated printing substrate.
The search for new methods of printing information and, more particularly, printing variable information goes on continuously. Recently the need to associate security features to methods of printing has gained a substantial importance, particularly as a result of the increasing concern that the business world is developing towards the monetary damages suffered from counterfeiting and grey marketing activities.
Having the above in mind, this invention provides a technology which renders direct printing integrating security features possible using a medium power laser beam typically of a few watts on a wide variety of substrates, provided the latter are coated using the coating scheme prescribed in this disclosure. The printing method and technology disclosed below becomes even more interesting in view of the possibility, well known in the trade, to control a laser beam direction and intensity in such a way that depending on the need the printed information may be a fixed and repetitive information or a variable information. Note that laser beams are currently already in use to inscribe variable information on paper or other substrates using other methods and technologies. For example, in the one well known case of desktop laser printers that have now become common office printing equipment, a low power laser beam of typically a fraction of a watt power installed in the printer is directed through appropriate controls to impart the desired information on to a photo-conductive surface in a way similar to the formation of a photo-image on the drum of a photocopier. The photoelectric image thus obtained is then transferred through a toner to the paper substrate that is originally placed in the tray of the laser printer. A second large class and already well known method of marking or information transfer to a substrate utilizing a laser is that wherein a medium to high power laser beam of several watts to kilowatts power is used. In this case the laser beam is directed to hit the substrate surface, the power is to be sufficiently high to cause the ablation of more or less minute quantities of the substrate surface material, thus leaving a visible trace. It is clear that a visible image will be left on the surface when the laser beam is controlled to hit the surface only at the spots which cumulatively constitute the desired final image. Such a control can be obtained either by using a high power laser beam of a few square centimeters cross sectional area that hits a mask where the desired image has been punched through, or with a single or multiple focussed set of beams of typically a few watts power where the single or multiple beams are controllably deflected in order to scan the surface of the substrate, to trace upon it the desired image, while simultaneously causing an ablation of the surface material by local melting and/or evaporation, the end result being obviously the formation of a visible image.
The above two well known laser printing methods have certain obvious limitations, such as in the first case, the printed surface is constrained to be essentially that of a printable grade fine paper sheet that can be fed into the printer; in the second case, the emanation of fumes or printing wastes that have to be continuously exhausted is a major concern. Finally and most importantly, the above laser printing methods do not lend themselves to date to the introduction of any security printing elements to the otherwise ordinary printing results.